The present invention relates generally to damping devices used in controlled damping applications such as semi-active vehicle suspension systems. More particularly, the present invention relates to high performance controlled damping devices using magnetorheological (MR) fluid.
High performance controlled damping applications, such as those used in passenger vehicle suspension systems, preferably provide a relatively low damping force at low speeds for comfort, and provide relatively high damping force at high speeds for safe handling of the vehicle.
One known semi-active suspension was introduced on General Motors cars and used continuously variable real-time damping (CV-RTD) actuators. The CV-RTD was based on pulse width modulation (PWM) of a two-stage pressure control valve that produces a desired damping force by using an electromagnetic solenoid to continuously modulate an armature plate. This actuator requires a triple tube construction and a complex valve design that results in a relatively expensive system that is rather sensitive to manufacturing tolerance. Previous analysis and test data showed that the stiffness of the deflective disk plays a key role in determining the dynamic stability of the CV-RTD valve. It was also not possible to achieve high frequency wheel control with the current CV-RTD based suspension system. It would be desirable to develop cost effective, high performance and robust RTD actuators using, for example, smart fluids (e.g., Electrorheological (ER) and Magnetorheological (MR) fluids) with controllable rheology and fixed flow area instead of moving mechanical valves with variable flow area. ER fluids require very high electrical fields (on the order of 5 kV/mm i.e., Kilovolt/milimeter) to produce the desired effects, whereas MR fluids produce similar effects at voltages well below 12V and hence are preferred for automotive use.
Magnetorheological (MR) fluids consist of magnetizable microparticles (e.g., iron and/or iron alloy powders) suspended in an inert base fluid (e.g., synthetic oil). MR fluids exhibit Newtonian Flow characteristics, with negligible yield stress, when there is no external magnetic field. However, the yield stress of a MR fluid can be increased by several orders of magnitude by subjecting it to a magnetic field perpendicular to the flow direction. This Bingham plastic behavior of MR fluid in the xe2x80x9conxe2x80x9d state is advantageous in creating actuators with controllable force or torque such as vibration dampers and clutches, without using any moving valves. MR fluids, and devices using the MR fluids, are well known. However, earlier problems with sedimentation and abrasion discouraged their use. Recent advances in material technology and electronics have renewed the interest in MR fluids for applications in smart actuators for fast and efficient control of force, e.g. damping, or torque in a mechanical system.
The damping performance of a MR fluid based CV-RTD damper is largely dependent on the force-velocity characteristics of the damper. FIG. 1 illustrates the optimum force-velocity characteristics of a damper used in automotive applications. The slope of the off-state force-velocity curve should be as low as possible for a smooth ride, with a desirable value of approximately 600 N-s/m(Newton-second/meter). The on-state force-velocity curve preferably has an initial slope in the range of 5-30 kN-s/m(KiloNewton-second/meter) up to a velocity of 0.1 to 0.4 m/s(meter/second) and a final slope similar to that in the off-state. The desirable maximum on-force should be limited to a suitable value (e.g., 4500N) at 2 m/s. The ratio of the damping force when the damper is in the on-state (on-force) to the damping force when the damper is in the off-state (off-force) at a given velocity is known as the turn-up ratio. It is desirable to have a turn-up ratio of at least 3 to 6 at a velocity of 1 m/s for good control of the vehicle chassis dynamics.
FIG. 2 shows a known monotube MR damper 10 having a piston 12 sliding within a hollow tube 14 filled with MR fluid. The piston 12 is attached to a hollow rod 18 that slides within a sealed bearing 20 at one end of the body of the damper 10. The piston 12 contains a coil 22, which carries a variable current, thus generating a variable magnetic field across a flow gap 24 between an inner core 26 and an outer shell or flux ring 28 of the piston 12. A bearing 30 having relatively low friction is disposed between the flux ring 28 and the tube 14. The flux ring 28 and the inner core 26 of the piston 12 are held in place by spoked end plates 32. Terminals 34 of the coil 22 extend through the hollow rod 18 and are provided with suitable insulation for connection to a source of electricity. One end 36 of the tube 14 is filled with inert gas which is separated from the MR fluid by a floating piston 38. The floating piston 38 and inert gas 36 accommodate the varying rod volume during movement of the piston. U.S. Pat. No. 5,277,281 discloses a similar MR damper.
FIG. 3 illustrates the force-velocity characteristics of the type of MR damper disclosed in FIG. 2. Clearly, in comparison to the preferred curves of FIG. 1, improvements in the force-velocity characteristics of conventional MR dampers are desirable. Although the above-described conventional MR dampers may perform adequately in certain applications, these devices do not achieve the required turn-up ratio and substantially stiction free performance near zero velocity for realistic automotive applications. For example, the conventional dampers often permit excessive flux leakage from the piston core into the piston rod and the cylinder disadvantageously reducing the average flux density, and creating an asymmetric distribution of flux in the flow gap, resulting in decreased performance and increased power requirements. Also, many conventional dampers create excessive turbulence in the flow of fluid through the flow gap thus decreasing damping performance. Therefore, there is a need for a more compact MR damper capable of more effectively and controllably damping motion.
The present invention is aimed at developing an MR fluid based continuously variable real-time damper that best approximates ideal performance requirements, while minimizing the damper size and power requirements.
This and other objects are achieved by providing a damping device comprising a hollow tube containing a magnetorheological fluid and a piston assembly slidably mounted in the hollow tube to form a first chamber positioned on one side of the piston assembly and a second chamber positioned on an opposite side of the piston assembly. The piston assembly includes a piston core, a substantially annular flow gap extending between the first and the second chambers and defining a flow path through the piston assembly, and a magnetic assembly adapted to generate a magnetic field extending across the substantially annular flow gap. The flow gap includes a first end positioned adjacent the first chamber and a second end positioned adjacent the second chamber. A laminar flow enhancing feature or means is mounted on the piston assembly and positioned adjacent the first and the second ends of the annular flow gap for enhancing laminar flow and minimizing turbulence in the annular flow gap. The laminar flow enhancing feature may include a respective flow opening positioned adjacent each of the first and second ends of the substantially annular flow gap. The respective flow opening has a funnel-shaped cross-section defined by an outer annular curved surface extending outwardly from the substantially annular flow gap toward the hollow tube and an inner annular curved surface extending inwardly from the substantially annular flow gap toward a longitudinal axis of the piston core.
The present invention is also directed to a damping device including a hollow tube, a piston assembly comprised of the piston core, substantially annular flow gap and magnet assembly as described herein above, and further including first and second magnetic flux leakage reduction devices formed of a non-magnetic material and positioned adjacent the first and second ends, respectively, of the flow gap. Each of the magnetic flux leakage reduction devices are positioned adjacent the substantially annular flow gap and securely connected to the piston core. The first magnetic flux leakage reduction device may include a first end plate while the second magnetic flux leakage reduction device may include a second end plate wherein the first end plate is connected to the piston core by a brazed connection. The damping device may further include a rod connected to the first end plate wherein the first end plate is positioned between the rod and the piston core to position the rod a spaced distance from the piston core. The rod and the hollow tube are preferably formed of a non-magnetic material. A piston bearing may be provided on the piston assembly between the outer surface and the hollow tube. The piston bearing is preferably positioned axially along the piston assembly entirely between an axial center of the piston core and one of the first and second chambers. The non-magnetic end plates may each include the inner annular curved surface while a flux ring positioned radially between the substantially annular flow gap and the hollow tube may include the outer annular curved surface. The non-magnetic end plates may be formed of either stainless steel or hardened aluminum. The non-magnetic end plate may include a flat plate and a non-magnetic nut secured to said piston core wherein the non-magnetic nut extends axially beyond an axial extent of the piston core. Flow bypass channels may be formed in both the piston core and the flux ring.